268 CALIFORNIA ACADEMY OF SCIENCES. 



to fall back on the theory of a tendency towards a uni- 

 form coloration, white in this case, varied by sexual 

 selection which may have produced the beautiful clear- 

 ness and softness of effect, and by the need for discrim- 

 inative marks to distinguish between closely allied forms. 

 The plumage of the young is much less uniform, in- 

 clining to be mottled and streaked, and indicating the 

 coloration from which the species of to-day have been 

 •evolved. 



Genus Circus. Marsh Hawk. 



(7) Male unlike female; young like female. 



Prevailing colors (of male) bluish gray, and white; 

 {of female) dark brown, tawny and white. 



How to explain the difference in the colors of the sexes 

 in this bird, except by an appeal to sexual selection, I 

 am unable to suggest, for the colors of the male are not 

 darker than those of the female, so that the law of katab- 

 olism could not be resorted to. The white rump-patch 

 and barred tail are excellent recognition marks, but of 

 what sort is difficult to determine. In reading the life 

 history of this species the use of so pronounced a recog- 

 nition mark is not made apparent. 



Genus Accipiter. Goshawks, etc. 



(2) Adult male similar to female; young like some 

 ancestral stage of adult. 



Prevailing colors, bluish gray, ashy, brown, rusty, 

 white. 



The difficulty of ascribing the colors of the birds of 

 prey to any form of utilitarian cause is again exhibited 

 in this genus. Can it be said that the bluish gray of 

 the back in adult birds is due to sexual selection, trans- 

 ferred by inheritance to the female? If not it seems 

 that an explanation must be despaired of for the pres- 

 ent, since for either protective or agressive resemblance 



